1. Field of the Invention
The present invention relates to a correction circuit for correcting errors in input data, and particularly relates to a data correction circuit suitable for correcting errors in a sequence of generally regularly and progressively incremented input data, such as digital time code reproduced from the timing track of a magnetic tape.
2. Brief Description of the Prior Art
In the video arts, when a video signal is recorded on a magnetic tape, audio signals, control signals, and time code signals corresponding to the video signal are normally recorded concurrently with the video signal on the tape. The time code signals are normally reproduced from the tape during a playback of the recorded video signal, and are especially useful in the electronic editing of the video signals. More particularly, the time code signals can be used to quickly find the beginning and/or end of a desired video sequence to be edited.
The time code signal is an orderly, incremental digital data signal whose value represents a frame number of the video signal and also indicates the time (hour, minute, and second) of the respective video frame. The time code signal is normally recorded on the magnetic tape in a separate track at the edge of the tape, but can instead be recorded together with the video signal during the vertical blanking interval of each field. The value of the time code signal increases (or decreases) by a predetermined amount for each frame. The value normally increases when the tape is moving in the forward direction during playback, and decreases when the tape is moving in the reverse direction. However, because this time code signal is relied upon for electronic editing, if an error occurs in the played back time code signal, difficulties will arise in the accurate electronic editing of the recorded video program recorded on the tape.
In order to avoid such problems, there has been previously proposed a circuit utilizing the regularity and the incremental nature of the time code signal to detect errors occurring in the played back time code data, and to correct such errors. Such previously proposed error correction circuits, an example of which is illustrated in more detail below, utilize a memory for storing data representing the played back time code data signal, a digital comparator for comparing the stored data with played back time code data, a switch coupled to the comparator and outputting the played back time code data when the latter has the same value as the data stored in the memory, but outputting the stored data when the latter and the played back code signal are unequal. The switch output is incremented and then written into the memory. Such previously proposed circuit also includes a control circuit for causing the switch to select the played back time code data as an output after a small predetermined number of successive errors (typically six errors) have been detected.
While the foregoing type of correction circuit is adequate in most circumstances, it does have at least the following drawbacks.
If dropout occurs in the time code signal, the number of successive errors in the played back time code signal may exceed the predetermined number of errors for which the control circuit is set. This can result in the time code data being incompletely corrected.
Furthermore, if the time code signal has been recorded on the magnetic tape by electronic editing, it may contain a "jump", that is, a discontinuity in the sequence of the time code recorded on the tape. However, the previously proposed correction circuit will provide its output signal delayed by a time corresponding to the number of errors which must be counted by the control circuit. If this number is a typical value, such as six, the jump in the output signal will be delayed by an error of six frames, and will result in a noticeable error in the reproduced video signal. Moreover, if the predetermined number of errors to be counted in the control circuit is selected to be a higher number of errors to accommodate dropout, the resulting error in the reproduced video signal will be even more noticeable.
While it is possible to arrange the control circuit to be adjustable, it is particularly troublesome to adjust the number of errors to be counted in the control circuit. Thus, the predetermined number is normally fixed at a relatively small value. Accordingly, with the previously proposed correction circuit, the errors in the played back time code signal are not completely corrected, and electronic editing of the recorded video signal is rendered somewhat difficult to accomplish.